Which latency do you mean ?
1) converter latency at the device for AD/DA conversion
2) latency by the driver between device and pc ?

to 1) this is only applicable for the Digiface, as the RayDAT is a pure digital card without ADDA section
to 2) RayDAT is directly connected to PCIe which is an advantage, Multiface had a PCI card connected (like a firewire card) but ran its own protocol over it.

When using a pure digital card like the RayDAT then you need to add the time for A/D and D/A conversion of the device which you connect via ADAT. RME documents this in their handbook in the section "Latency and Monitoring". The Latencies differ with sample frequency. The values below are all at 44.1 kHz.

Right beside the calculation the according screenshot from the manual, in one case for the UFX, in the other case for UFX and UFX+.

I used an UFX in front of a RayDAT: https://www.tonstudio-forum.de/blog/ind … ME-RayDAT/

This is how you calculate the total latency for such a setup:

See here an overview how different RME solutions, that I use compete against each other
in terms of reported RTT latency as reported by the driver to DAW:

As you can see, the UFX+ with latest converters and new MADIface driver has an even slightly lower latency compared to the combination of RayDAT and UFX.

To answer your question which of the two is better in terms of the i.e. "total latency", like shown in the calculations above, one would need to know which Preamp and/or AD/DA converter you plan to connect to the RayDAT. And then its should be a RME device, because other vendors do not communicate so open the technical data as RME does in their handbooks.
And you need of course a running Multiface solution to be able to read the RTT reported by the driver.

So .. not so easy to answer and it depends, what you connect behind the RayDAT via ADAT.
The RayDAT is definitively a very nice and reliable card.

But you could also take an UFX II or UFX+ if USB based should be your preference.

No, but it depends on what you call a difference.
On my PC I  usually work at 128 samples, 64 is also workable on my system but latency wise I can play fine up to 256. So at 64 or 128 I don't mind a difference of 0.1 or 0.5ms. So unless you specify what you are looking for (and maybe why), it is tough to answer.

But AFAIK all HDSP and hdspe's have the same driver and thus latency. PCIE has a higher bandwidth so can in the end support more channels, but are not faster.

I see! Well that is a really difficult one. Could be a number of things.
-Your system is not powerful enough for your load
-not tweaked enough
-wrong hardware (some mobo's will never work right at super low buffers)
-Some VSTi  and VSTfx don't like low buffers (for example Hreverb load drops enormously at 1024)
-And very maybe PCI vs PCIe will make a difference but I would not expect it. Only try it if you can return it.
If it works ok at 128 keep it at that and make sure none of your plugins add extra latency, which the daw will compensate and will thus add latency to the live playing too.

You don't mention what system and not even OS....

I assumed windows, which needs to be tweaked a little. And since you have a hdspe card (I missed the E)  the raydat will make zero difference.
And it is worth looking into the plugins you use. For example some Kontakt scripts don't like really low latency. Some pianos need 8 or more voices per note. For live you can choose with some care and reduce the load. Unless you mean live in the studio then my advice makes no sense...
I don't know how efficient logic is with live low latency. You could try reaper, it is free to try.

It's only one plugin that gives me the problem. It's a really nice Piano inst called EightyEight 2 by SONiVOX. And yea, I mean live in the studio.